I have come across this before in a post.

I am curious if this dog behaviour occurs during school hours or just in the privacy of home.

If the child behaves this way only in the privacy of home and not in public, then he can control the behaviour.

When he dresses in the opposite gender clothing and does makeup and nail polish, does he continue with the dog behaviour. Or are these two separate patterns of behaviour?

Does your nephew ever explain why he is acting like a dog? If an actual dog was in the household, I wonder what your nephew do?

A person with schizophrenia felt that she was treated like a dog, so she would act like a dog. There were two incidences in that person's life that she acted like a dog for extended periods of time. I have not come up with much more, just a performance artist who acted like a dog for his art.